Not sure if it has the hotkey support you want, but I've always liked ReText for its preview capabilities.
It's a bit heretical to mention, but lately, I've gotten to use Visual Studio Code for almost all text editing I do, it might suit you too.
Consider Retext. I use it to write documentation using Markdown. It has neat "Live Preview" feature that allow you to instantly see rendered results of the formatting you use.
> if you are still interested there are some themes here https://github.com/a-mt/retext-themes and activating exts https://github.com/retext-project/retext/wiki/Markdown-extensions
The issue isn't themes or Markdown-extensions. The issue is lack of basic keyboard shortcuts. Like in most editors you can delete a line by Ctrl + K or Ctrl + Shift + K (vscode). This is just one example. Things like ability to move lines, duplicate lines etc. are essential to me. Basic text editing functionality is what is missing in Retext.